1. Field of the Invention
This invention relates generally to a video display system, and more particularly to such a system which includes a flat panel of X-Y matrix type and an improved brightness control circuit for driving the panel.
2. Description of the Prior Art
Recently, video display systems utilizing a flat panel of X-Y matrix type have become the object of considerable interest.
In such systems, different kinds of flat panels, such as, gas discharge panels, liquid crystal panels, electroluminescent panels, and the like have been used, and extensive research has been conducted in respect to the flat panels and their driving circuits.
However, the video display systems of the prior art are still unable to reproduce pictures with good quality on the flat panel.
One of the sources of the difficulties is to provide a brightness control circuit which responds to changes in the level of the video signal with sufficient accuracy. In more detail, the brightness control of the panel used in the prior art is usually achieved by varying either the amplitude or the duration of the current or driving pulse of the display cell of the display panel. These two methods are respectively referred to as "PAM" for pulse amplitude modulation and "PWM" for pulse width modulation. Inherent in the "PAM" method is the difficulty in obtaining driving transistors with sufficiently uniform characteristics to achieve brightness control with good quality for a substantial range of different pulse amplitudes.
On the other hand, the "PWM" method is disadvantageous in that a sufficient number of steps or grades of brightness cannot be obtained because of the time delay of the display panel.